The invention relates to a method of producing carbonaceous bodies, in particular electrodes of parallelepipedal, cubic, or cylindrical shape, such as, for example, electrodes for the production of aluminum or steel, by calcining raw shaped bodies containing pyrolyzable substances, in a tunnel type furnace, wherein the volatile organic distillation products released from the binder after the pre-heating zone in the temperature range of 200.degree. to 600.degree. C. and the gaseous or particulate decomposition products are completely burned off.
The raw shaped bodies, also referred to as the carbonaceous bodies, are generally formed from a mass of petroleum coke, anthracite, carbon black, graphite, or the like, additivated with a binder, such as tar and/or pitch. In the combustion of such shaped bodies, the binder is cokefied to provide desired electrical and mechanical properties.
The description that follows will be given with specific reference to the process for the obtainment of electrodes useful in the production of aluminum, it being understood that the same method, with some appropriate modifications, may also be applied to the obtainment of electrodes for steel production.
Known are methods for the obtainment of electrodes for the production of alumium wherein the electrodes are calcined on an industrial scale in annular open chamber furnaces or in tunnel type furnaces. Tunnel type furnaces, such as disclosed for example in published patent DE-OS No. 30.42.708 or in EP-OS No. 0.103.130, afford the following distinctive advantages:
high reproducibility of the calcining conditions; PA1 improved hygiene of the working area; and PA1 efficient thermal recovery.
Tunnel type furnaces for aluminum production anodes comprise in principal four zones: a pre-heating zone (up to a temperature of the carbonaceous bodies of about 200.degree. C.), a combustion zone (up to a temperature of the carbonaceous bodies of about 600.degree. C.), a firing or calcining zone (up to a temperature of the carbonaceous bodies of about 1000.degree.-1200.degree. C.), and a cooling zone.
In the calcining process with tunnel type furnaces, a series of trucks, as previously loaded with raw carbonaceous bodies are pushed in, with the truck speed in the range of 0.5 to 3 meters per hour. The shaped bodies are preferably embedded in a powder filler containing carbon for protection against oxidation and deformation, and placed into muffles carried on the trucks.
The trucks are pushed through the furnace in accordance with a preset temperature pattern, also referred to as the firing curve. The temperature pattern, respectively the heating rate resulting therefrom, is a determining process parameter in the production of high quality fired electrodes. Once the firing curve of the tunnel type furnace is set, all the carbonaceous bodies are calcined in the same way, which affords a high degree of reproducibility. The trucks pushed through the tunnel type furnace have for example a loading surface of 5.times.4 meters, and one or more layers of the carbonaceous bodies may be laid onto a truck.
Calcining carbonaceous bodies in tunnel type furnaces has posed a problem which is but marginal or not encountered at all with annular chamber furnaces: the formation of cracks. This is to be attributed to physical and chemical changes undergone by the binders during calcination.
As mentioned above, carbonaceous bodies to be calcined are immersed in a powder filler containing carbon, usually in a box of a ceramic material which can withstand the high firing temperature. This is on the one side expensive from the standpoint of capital cost, and on the other side lowers the thermal efficiency of the tunnel type furnace because the carbon-containing powder filler acts simultaneously as a heat insulator.